In a typical electrostatographic copying machine, a photoconductive surface has a uniform charge applied to it. The surface is then imagewise exposed to light to selectively discharge the surface through a grounded conductive layer, leaving behind an electrostatic latent image on the photoconductive surface. This latent image is developed with toner particles to form a visible image. The visible image is transferred to a receiver sheet to which the visible image is permanently fixed by fusing it with heat and/or pressure. To prepare the photoconductive surface for the next copying cycle, the surface is cleaned by a cleaning station. This cleaning station removes debris, such as background toner, dirt and receiver sheet fibers, from the surface.
The step of developing the latent image may be performed with a magnetic brush. Such a brush includes a core composed of a series of radially mounted, alternating pole magnets. The magnets are surrounded by a cylindrical shell made of a nonmagnetic, conductive material. Relative movement is provided between the core and the shell by rotating the core and/or the shell. Developer material, made up of magnetic carrier particles and toner particles, is applied to the shell. The toner and carrier particles are triboelectrically charged to opposite signs and are thus attracted to each other. The developer material adheres to the shell due to the carrier particles' attraction to the magnetic core.
To control attraction of the toner particles to the latent image on the photoconductive surface, an electrical bias is applied to the shell. This bias sets up an electric field between the shell and the latent image. The toner particles, under the influence of the electric field, leave the shell and adhere to the photoconductive surface in a pattern corresponding to the latent image. This creates a visible image of toner.
If the shell is rotated, one method of applying the electrical bias to the shell is through the use of a brass brush connected to a voltage supply. The brass brush has a 3/8" bristle and bears directly on the shell surface. A problem with this setup is that the brass brush becomes contaminated with developer material. The brush wears out quickly in the abrasive atmosphere of the developer material, necessitating frequent replacement of the brush. Additionally, as the developer material is somewhat electrically insulative, it can hinder the electrical contact between the brush and the shell.
Another method of applying the electrical bias to the shell is to apply it through metal gears used to drive the shell. The bias is applied to a motor-gear (held by a suspended plastic arm) through its bearing and shaft. The motor-gear meshes with a gear mounted on the shell. The disadvantage of this system is that, as the gears rotate, the electrical path becomes broken when the gears mesh and when backlash occurs.